1. Field of the Invention
The present invention relates to an optical cable laid in the underground, on the ground, in the air, or on the sea bottom; and a chamber element utilized when optical fibers are mounted in the optical cable.
2. Related Background Art
Conventionally known as a technique in such a field is that disclosed in Japanese Patent Application Laid-Open No. 55-45087. The optical cable disclosed in this publication comprises a plurality of chamber elements made of a plastic each provided with a fiber containing cavity for containing a coated optical fiber, a loose tube, a ribbon fiber, or the like (hereinafter simply referred to as "optical fiber" in general terms); and a tension member functioning as a central member. The individual chamber elements containing optical fibers are assembled around the central member in the state where their bottom faces are in contact with the central member. Each chamber element has a bottom part and a pair of side wall part parts rising from both ends of the bottom part. Namely, the chamber element has substantially a U-shaped cross section. The chamber element is extrusion-molded as a straight elongated member. It is considered preferable that the bottom part and side wall parts of the chamber element each have a thickness of 0.5 mm or greater.
Also, Japanese Patent Application Laid-Open No. 4-182611 discloses an optical cable in which a plurality of chamber elements each having substantially a U-shaped cross section and containing an optical fiber therein are assembled in an S-Z strand around a central member. In this case, as shown in FIG. 6, in order for fiber containing cavities 102 to face outward, chamber elements 100 are assembled around the central member 110 in the state where their bottom faces are in contact with the central member 110. Accordingly, in the S-Z locus formed by each chamber element 100, at each position Re (hereinafter referred to as "S-Z reverse portion"; see FIG. 7) where S strand turns into Z strand or vice versa, it is necessary to bend the chamber element 100 in the widthwise direction x of its fiber containing cavity 102 as shown in FIG. 8.
The conventional optical cables and the chamber elements therefor, however, have the following problems. Namely, in the conventional chamber elements, each of the bottom part and side wall part parts has been formed with a uniform thickness. Also, the thickness of the bottom part and that of each side wall part have been equal to each other. Accordingly, when a chamber element is forcibly bent in the widthwise direction x of the fiber containing cavity in order to form the S-Z reverse portion Re, its inner side wall part 101a (on the curvature center side) leans outward (toward the fiber containing cavity 102) as shown in FIGS. 9 and 10. On the other hand, the outer side wall part 101b leans inward (toward the fiber containing cavity 102). Consequently, thus leaning side wall parts 101a, 101b may collide with a ribbon fiber stack 103 contained in the fiber containing cavity 102, thus applying a lateral pressure to the ribbon fiber stack 103 (see FIG. 10). As a result, transmission loss in each ribbon fiber 104 of the ribbon fiber stack 103 may increase.
Also, when both side wall parts 101a, 101b lean toward the fiber containing cavity 102 and come close to each other, the opening of the fiber containing cavity may be blocked by the side wall part parts 101 as shown in FIG. 10. Consequently, in the case where, after the optical cable is laid, the ribbon fibers 104 are to be taken out from the optical cable so as to branch out, it becomes difficult to take out the ribbon fibers 104 from within the chamber elements 100, thus lowering workability.